This invention relates generally to the measurement of energy transmission through an aperture pattern in a longitudinally moving strip of material and particularly to the measurement of light through the aperture patterns in shadow masks for kinescopes.
In the manufacture of shadow masks for color television kinescopes, a roll of flat material is coated with a photoresist material and is subsequently photoexposed to form a series of aperture patterns, and the peripheries of the shadow masks, in the photoresist material. The unexposed photoresist material is washed away leaving bare metal. The strip of material is subjected to an acid etching process in which the bare metal is removed to form the apertures, and partially etched peripheries used to remove the shadow masks from the strip of material. After the etching is completed, the transmission of energy through the aperture pattern is measured to verify that the shadow masks are suitable for the intended use. The measurement of light transmission through the aperture patterns typically is accomplished by shining a known intensity of light through the apertures and noting the percentage of light which passes through the apertures. The acid etching and the light transmission measurements typically are made while the strip of material is pulled longitudinally along a conveyor line. Because of the long length of the strip, there is a tendency for the strip to move transversely back and forth perpendicular to the longitudinal motion. The areas of the apertures vary along the transverse dimension of the pattern and for this reason, accuracy of the light transmission measurement requires that all measurements be made at the same transverse position of the aperture pattern. Accordingly, transverse motion causes a degradation of the measurement accuracy. It is extremely difficult, if not impossible, to restrain the long strip to prevent the transverse motion.
One technique of overcoming the adverse effects of the transverse motion includes movably mounting the measuring device to track the transverse motion and maintain a substantially constant relative position between the measuring device and the aperture pattern. This technique suffers the disadvantage of being complex and expensive. Therefore, there is a need for a system in which the sensor is permanently mounted and the undesired motion is compensated.